1. Field of the Invention
The invention relates to a bipolar transistor, and more particularly to a heterojunction bipolar transistor including a compound semiconductor therein, and also relates to a method for fabricating such a bipolar semiconductor.
2. Description of the Related Art
A bipolar transistor has advantageously a greater ability for driving electrical current than a field effect transistor (generally referred to as "FET"). For this reason, recently a bipolar transistor using therein a chemical compound semiconductor such as GaAs as well as Si has been researched and developed. In particular, a bipolar transistor using a compound semiconductor therein has many advantages, one of which is that an emitter-base junction can be constituted of a heterojunction and hence emitter injection efficiency can be maintained to be high regardless of a base having a higher density. Accordingly, such a bipolar transistor is presently being researched for driving a simplex element at a high speed, applying to various circuits and the like.
In order to achieve a higher performance of a simplex element using therein such a heterojunction bipolar transistor (hereinafter referred to as "HBT"), or a circuit to which such a simplex element is applied, it is critical to decrease a base resistance and also shorten base transit time.
It is advantageous to reduce a contact resistance in electrodes in order to reduce a base resistance. For this end, there has been suggested a method in which a heavily C-doped layer is selectively regrown in an extrinsic base area. Such a method is disclosed, for instance, in Japanese Public Disclosures Nos. 4-83345 and 4-83346 and a report authored by Shimawaki et al. by the title of "AlGaAs/GaAs HBTs with heavily C-doped extrinsic base layers selectively grown by MOMBE" reported in "The Institute of Electronics, Information and Communication Engineers" Vol. 92, No. ED 92-134, 1993, page 23.
FIG. 1 is a schematic cross-sectional view of a bipolar transistor disclosed in the above mentioned report. A semiconductor chip in FIG. 1 includes a semi-insulating substrate 1 composed of GaAs, a collector contact layer 2 (3 * 10.sup.18 cm.sup.-3, 500 nanometers) composed of n-GaAs, a collector layer 3a (5 * 10.sup.16 cm.sup.-3, 400 nanometers) composed of n-GaAs, an intrinsic base layer 5b (4 * 10.sup.19 cm.sup.-3, 80 nanometers) composed of p-GaAs, an emitter graded layer 6 (3 * 10.sup.17 cm.sup.-3, 20 nanometers) composed of n-Al.sub.x Ga.sub.1-x As (x is varied in the range of 0 to 0.25), an emitter layer 7 (3 * 10.sup.17 cm.sup.-3, 150 nanometers) composed of n-Al.sub.0.25 Ga.sub.0.75 As, a graded layer 8 (3 * 10.sup.17 through 6 * 10.sup.18 cm.sup.-3, 50 nanometers) composed of n-Al.sub.x Ga.sub.1-x As (x is varied in the range of 0.25 to 0), a n-GaAs layer 9 (6 * 10.sup.18 cm.sup.-3, 80 nanometers), a graded layer 10 (2 * 10.sup.19 cm.sup.-3, 50 nanometers) composed of n-In.sub.x Ga.sub.1-x As (x is varied in the range of 0 to 0.5), an emitter contact layer 11 (2 * 10.sup.19 cm.sup.-3, 50 nanometers) composed of n-In.sub.0.5 Ga.sub.0.5 As, an extrinsic base layer 12a (4 * 10.sup.20 cm.sup.-3) composed of p-GaAs, an emitter electrode 13 composed of WSi, a base electrode 14 composed of Ti/Pt/Au, a collector electrode 15 composed of AuGe/Ni/Au, an electrode 16 composed of Ti/Pt/Au for taking out an emitter therethrough, a SiO.sub.2 layer 17, 18, 19 and an insulation area 20.
In FIG. 1, the p-GaAs layer 12a is formed by selective regrowth using a metalorganic molecular beam epitaxy method (generally referred to as "MOMBE"). In the p-GaAs layer 12a, carbon, which is a p-type impurity, is doped heavily or in high density, and the intrinsic base layer 5b is formed to be uniform base structure.
On the other hand, in AlGaAs/GaAs HBT not using selective regrowth for forming an extrinsic base area unlike MOMBE, a p-AlGaAs graded layer is used as a base layer for shortening base transit time. The p-AlGaAs graded layer has continuous gradient with increasing concentration of Al from a base-collector junction towards a base-emitter junction. Since electrons, which are minority carriers, traveling across a base layer are accelerated by pseudo electric field in this base structure, this base structure can shorten a base transit time and enhance a current gain relative to the above mentioned uniform base structure in which electrons run in a base layer by diffusion effect.
It is necessary to prevent aluminum oxide from growing at a regrown interface between the intrinsic base layer 5b and the extrinsic base layer 12a, in order to reduce a base resistance in HBT in which an extrinsic base area is to be formed by regrowth as aforementioned. Accordingly, it is desirable to constitute an intrinsic area of a uniform base structure composed of GaAs, but undesirable to constitute an intrinsic area of a continuous gradient having base structure composed of an AlGaAs graded layer. Thus, it has been almost impossible conventionally to reduce a base resistance and also shorten a base transit time by means of the above mentioned regrowth method.